1. Field of the Invention
The invention is generally related to the area of integrated circuits designs. More particularly, the invention is related to designs of one or more capacitors in integrated circuits without requiring additional spaces, wherein the capacitors are integrated with inductive components (e.g. inductors or transformers) and without affecting the inductive components.
2. The Background of Related Art
An integrated circuit (IC), sometimes called a chip or microchip, is a piece of semiconductor wafer on which thousands or millions of tiny components, such as resistors, capacitors, and transistors, are fabricated. An IC can function as an amplifier, oscillator, timer, counter, computer memory, or microprocessor.
A capacitor is a passive electronic component that stores energy in the form of an electrostatic field while an inductor is a passive electronic component that stores energy in the form of a magnetic field. In its simplest form, a capacitor consists of two conducting plates separated by an insulating material called the dielectric and an inductor consists of a wire loop or coil. Capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separation between the plates. Capacitance also depends on the dielectric constant of the substance separating the plates. Inductance is directly proportional to the number of turns in the coil. Inductance also depends on the radius of the coil, the space between the turns, thickness of the material of the coil, and on the type of material around which the coil is wound. For a given coil radius and number of turns, dielectric materials such as wood, glass, and plastic result in the least inductance while ferromagnetic substances such as iron, laminated iron, and powdered iron increase the inductance. The shape of the core as well as the wire or coil can also be significant. The standard unit of capacitance is the farad, abbreviated F. Farad (F) is a large unit; more common units are the microfarad, abbreviated μF (1 μF=10−6 F) and the picofarad, abbreviated pF (1 pF=10−12 F). The standard unit of inductance is the henry, abbreviated H. Henry (H) is a large unit. More common units are the microhenry, abbreviated μH (1 μH=10−6 H) and the millihenry, abbreviated mH (1 mH=10−3 H). Occasionally, the nanohenry (nH) is used (1 nH=10−9 H). As signal frequency goes high, for example, in gigahertz range, inductors with the picohenry (pH) are often used (1 pH=10−12 H).
Capacitors can be fabricated onto integrated circuit (IC) chips. They are commonly used in conjunction with transistors in dynamic random access memory (DRAM). The capacitors help maintain the contents of memory. Capacitors are sometimes used with inductors or transformers in various applications such as wireless or high speed data communications. A capacitor connected in series or parallel with one or more inductors can provide discrimination against unwanted signals. FIG. 1 shows a circuit 100 preferably implemented in an integrated circuit and is used for a bridged-T termination network providing constant input impedance with a capacitive load. The circuit 100 includes two capacitors, two inductors and three resistors, each with different values, to achieve characteristic impedance over a wide frequency range even when feeding into a capacitive load.
The traditional approach to implement capacitors that are designed to work in conjunction with inductors is to implant pairs of conducting plates in a semiconductor material. The size of the plates depends on the required capacitance. FIG. 2 shows a possible layout (not scaled) of an implementation of the circuit 100. As shown, each of the components (two capacitors, two inductors and three resistors) is implemented. Because of the possible interference from the inductors, there are some requirements about the implementation of the inductors in presence of other components. One of the requirements (e.g., in the process of Low Temperature Cofired Ceramic or LTCC technology) is that no other components shall be within a certain range of the inductors, resulting in occupation of more silicon space. However, in many circuit designs, especially, high-speed circuits, capacitors are often used in conjunction with inductors. The traditional approaches, as one shown in FIG. 2, would take a significant space in a silicon chip to implement capacitors with usable capacitance.
FIG. 3 duplicates FIG. 6 of Galal et al, “Broadband ESD Protection Circuits in CMOS Technology”, IEEE Journal of Solid-State Circuits, Vol. 38, No. 12, December 2003, pp. 2334–2340, which is hereby incorporated by reference. FIG. 3 shows an input electrostatic discharge protection circuit that involves more capacitors and inductors used together. When a chip includes a large number of inductors and capacitors, especially with varying capacitance, the cost of the chip can be a factor of the number of the inductors and the capacitors or the total area of the conducting stripes for the inductors and capacitors.
There is thus a tremendous need for solutions of implementing on-chip capacitors in conjunction with inductors or transformers without taking up too much wafer space and, at the same time, without compromising functions or performance of the inductors or transformers. The present invention provides designs of capacitors integrated with inductors or transformers and does not take up additional spaces in a silicon wafer and affects inductance of the inductors or transformers thereon.